Cut big mesh (plane) in quadratic parts
To do this first join all your objects into one if they aren't already by selecting them all and hitting Ctrl+J.  Then create another grid object consisting of just edges which you will use to slice up the map.



Select the grid and then the terrain (in that order), enter edit mode, go to top view (numpad 7), make sure you are in orthographic mode (numpad 5 to toggle), then use the knife project tool found in the toolbar.  Make sure cut through is checked after you do so.



Your terrain will then be sliced up based on the grid object's edges.